Switching regulators are intended to be efficient machines for converting an input voltage to an output voltage. The two most common types of switching regulators are Boost (voltage increasing converters) and Buck (voltage decreasing regulators). Both Boost and Buck regulators are very important for battery powered applications such as cellphones.
As shown in FIG. 1A, a traditional implementation for a Boost regulator includes an inductor L connected between an input voltage (VBATT in this case) and a node VX. A switch M1 is connected is connected between the node VX and ground. A diode D is connected between VX and the output node (VOUT) of the regulator. A filtering capacitor connects VOUT to ground. A control circuit turns switch M1 on and off in a repeating pattern. This causes the Boost regulator to have two distinct operational phases. In the first phase, shown in FIG. 1B, the switch M1 is on. During this phase, called the charging phase, the inductor is connected between the battery and ground. This causes the inductor L to store energy in the form of a magnetic field.
In the second, or discharge phase the switch M1 is opened (see FIG. 1C). In this phase the battery, inductor and diode are connected in series with the load. As a result, current flows to the load as the magnetic field previously stored by the inductor collapses. The series connection of the battery and inductor means that current is delivered at greater than battery voltage. As the inductor's magnetic field collapses and the voltage over the inductor falls, the diode prevents current at the load from actually reversing.
The regulator architecture just illustrated suffers one fundamental flaw: the diode D has, by nature a forward voltage drop. Depending on the type of diode, this can be fairly small, but is still generally unacceptable for low voltage applications. For this reason, it is common to replace the diode D with a second switch M2. FIGS. 2A and 2B show Boost and Buck regulators of this type, respectively.
The basic idea is that the switch M2 operates with no voltage drop (when switched on) overcoming the disadvantages inherent in diode based designs. Regulators of this type are often referred to as “synchronous regulators” because the two switches are driven synchronously—when one is on, the other is off. In the real world, this is never quite the case. It takes time to turn the switches on and off and control cannot be done with absolute precision. For this reason, the act of turning a switch off is always done slightly in advance of the act of turning the other switch on. This technique, known as break-before-make or BBM avoids the situation where both switches are on at the same time and power is connected to ground (a condition known as shoot through).
In general, switching regulators work in environments where both the input and output voltage are dynamic. Input voltages change as battery voltages decline over time or as other components draw more power. Output voltages change depending on load requirements. Switching regulators react to changes in input and output voltages by varying the amount of time that the switches M1 and M2 remain on. This is done using two different methods. In the first method, the switching frequency is varied—as the load on the regulator increases (relative to its supply) the switching frequency is increased. This is known as pulse frequency modulation or PFM. In the second method a fixed switching frequency is used and the amount of time that the switches M1 and M2 are turned on is varied. For larger loads, the switches stay on longer. This is known as pulse width modulation of PWM. Of the two methods, PWM is often preferred because it produces noise at a known and therefore filterable fixed frequency. Filtering the noise created by a PFM converter can be problematic—especially in portable applications.
Switching regulator operation may be “continuous” or “discontinuous.” In continuous operation no allowance is made for reverse current flowing backwards from the output to the input. This mode suffers losses due to the ripple reverse current flowing in the inductor, which can be rather high for small inductors and higher voltages. This loss becomes more noticeable at lower output current.
One technique for reducing this type of loss is to use a comparator to sense the current in one of the switches (the high-side switch for boost converters and the low-side switch for buck converters) and turn off that switch when the current starts to flow backwards (i.e., from the load to the regulator). This reduces the ripple losses and reduces ripple current at light loads.
There is another benefit to the reverse current comparator: it means that no matter how hard one may try, there is always a small current emerging from the regulator because there is a minimum on-time required in a current mode regulator operating in normal mode. In a no-load condition this small current will continue to increase the voltage on the output capacitor until an overvoltage condition is sensed. At that point, various power saving strategies may be invoked. For one of these the regulator is turned off and put to sleep until the output voltage falls within regulation. This strategy is often used with PFM type converters and results in what is called “burst mode” characterized by periods in which the regulator switches rapidly separated by idle periods of varying duration. PWM converters can implement another power saving strategy by deciding at the beginning of each switching cycle whether output voltage is in regulation. If an overvoltage condition exists, the entire switching cycle can be skipped. This strategy, known as “pulse skipping” is characterized by constant frequency switching periods separated by idle times that last for some multiple of the basic PWM cycle. Regulators can also switch between PWM for normal operation and PFM for light load operation.
In addition to preventing current from flowing from the load to the regulator, the reverse current comparator may be used to detect light load conditions and allow the regulator to adopt a light load strategy. In practice, there are two main requirements for this comparator, that it be fast and accurate. In addition, is should consume little current. Typically the comparator is measuring millivolts in a nanosecond environment. This is very difficult to do, normally because of the competing requirements of speed and accuracy. For this reason, there is a need in the prior art for improved comparator designs.